The present invention generally relates to an indicating instrument used in connection with an RF transmission system to indicate the VSWR of the system. More particularly, the invention relates to an antenna VSWR indicator which allows the VSWR of an antenna in an RF transmission system to be accurately indicated such that the antenna can be properly matched or coupled to an RF transmitter at the frequency in use.
In an RF transmission system, which may include a radio transmitter and antenna system, the power of an RF signal applied to the transmission line from the RF transmitter to the antenna may not be completely absorbed by the antenna or other transmitting apparatus. If- the antenna impedance is not properly matched or coupled to the transmitter impedance at the frequency in use, a portion of the RF energy transmitted through the RF transmission line may be reflected back along the transmission line so as to reduce the power transfer or create disturbances in the transmission system. The terminating RF load, such as an antenna, must be properly matched to the RF source, such as an RF transmitter, to assure maximum power transfer and maximum efficiency in the RF transmission circuit.
Upon the occurrence of an improper match of the antenna impedance to the characteristic impedance of the transmitter the reflected energy will generate standing waves on the transmission line. The match between an RF transmitter and antenna may be determined by measuring the power flowing into the antenna as well as the reflected power, if any, flowing back from the antenna along the transmission line therebetween. The ratio of the power flowing into the antenna and the reflected power is called standing wave ratio (SWR) or voltage standing wave ratio (VSWR). For example, an antenna resonant at a given operating frequency will exhibit an impedance of near 50 ohms corresponding to a transmitter impedance of 50 ohms, thereby allowing no reflected power, and giving a VSWR of 1:1. An improperly tuned antenna will cause values of reflected power to show VSWR's of 2:1; 3:1; 4:1; open or short conditions which will degrade overall system performance and reduce the signal strength of the radiated signal. Additionally, any reflected power returned to the RF source along the transmission line could damage or destroy a source such as an RF transmitter.
The phenomena of voltage standing wave ratio (VSWR) is usually defined in terms of the ratio of reflected power and the transmitted power which defines a voltage reflection co-efficient (P). The quality of an impedance match between the RF transmitter and antenna may be defined according to the formula: ##EQU1##
Various prior art measurement instruments have been developed for measuring VSWR or giving an indication of the VSWR for a particular Rf transmission system. For example, directional watt meters may be used to measure the forward traveling signal indicating the transmitted power across the transmission line. Similarly, the instrument may be used to measure the reflected signal indicating the reflected power, wherein the relationship between the transmitted and reflected power could then be analyzed to obtain the power ratio necessary to compute the VSWR of the system. A combination of two directional measurement devices could be used to measure the net power delivered by the resultant signal to an ordinary load, thereby giving an indication of the VSWR of the system. As an example, in U.S. Pat. No. 4,110,685 a standing wave ratio measuring instrument includes two directional couplers which are inserted into the transmission line to produce output voltages indicative of the forward and reflected RF power. The measured voltages are processed to generate a signal proportional to the return loss in the system, which is related to the VSWR of the system. This signal is coupled to an analog display meter which is calibrated to provide a direct reading of the VSWR. Other similar VSWR measurement devices also measure the relative magnitude of the incident and reflected voltages to derive an indication of the VSWR of the transmission system. Examples of such systems can be found in U.S. Pat. Nos. 3,683,274 and 3,020,529.
In measurement instruments of the type found in these prior patents, the directional couplers used in the systems may be current transformers which are used to derive the forward and reflected voltages traveling in the transmission line. Such current transformers may cause these shifts of unknown magnitude, which are especially apparent at low frequencies. Such phase shifts introduce error into the resultant measured voltages and limits the accuracy of the VSWR measurement. Similarly, at high frequencies, a phase delay may be induced into the secondary winding of a transformer which again may limit the accuracy of the measurement therefrom. The frequency bandwidth may thereby be limited by the directional couplers used in these measurement systems which will limit their effective use. It has additionally been found that such measurement instruments are simply inserted at a point along a transmission line between the transmitter and antenna, and rely upon the transmitter as the source of RF power therefore. Relatively high RF power levels may be needed to obtain proper VSWR measurements, which present significant hazards to the user while tuning or adjusting an antenna of the system. There also exists the possibility that, if the VSWR is too high, the power reflected back to the transmitter could affectively destroy the output stage of the transmitter.
Other VSWR measurement instruments have been developed which avoid some of the problems associated with the measurement instruments as described above. In U.S. Pat. No. 4,580,092, a portable antenna match indicator for VSWR measurement is described which provides a direct readout of VSWR for the antenna. This instrument includes its own internally tunable low level RF signal source and is connected to the transmission system antenna for testing. Again, directional couplers are connected between the RF signal source and the antenna so as to sample the forward and reflected power on the transmission line coupling the two. Although the antenna match indicator as disclosed in this patent avoids reliance upon the RF transmitter as the source of RF power, significant disadvantages exist with respect to the frequency bandwidth over which the device can accurately measure VSWR.
There have also been developed in the prior art, a VSWR measuring device termed a "RX box" utilizing variable resistors and capacitors in the measuring circuit which are adjusted to null the received signals, thereby giving an indication of the VSWR for the system. A drawback of this type of device is found in that knowledge of various parameters of the antenna or other RF load were required along with the necessity for calculating the measured VSWR using these parameters along with the measurements obtained. Such a system does not provide an instantaneous indication of the VSWR, and is prone to error resulting from incorrect calculation or knowledge of the antenna parameters. Also, various measuring circuits designed to yield an indication of VSWR have included the use of bridge circuits to measure the reflection co-efficient of a transmission line network. The bridge circuit provides a reference against which an unknown impedance, being the impedance of the antenna or other RF load, is compared to give an indication of the VSWR. In prior art designs using a bridge circuit, an RF noise source has been provided by a noise diode and amplifier circuit to energize the bridge. Such bridge circuits have imposed a limit on the achievable accuracy of the measurement and/or have been frequency dependent and bandwidth limited due to the RF noise source only having useable harmonics up to about 50 MHz. Also, the use of a bridge circuit will normally require referencing one side of a response curve for the RF transmission system, wherein as the reference is approached the signal will be nulled. In this situation, it is possible that a detected VSWR would not be accurately indicated if on the other side of the response curve, unless the bridge circuit was perfectly balanced which is normally not the case.